Voltage-gated sodium (Na) channels initiate and conduct action potentials in neurons and are a molecular target for anti-epileptic drugs. Our work on this project has shown that deletions of both a and subunits of Na channels cause seizure susceptibility and epilepsy in mice. These results show that alterations in the expression and cell biology of Na channels can lead to epilepsy without gain-of-function mutations. Human Severe Myoclonic Epilepsy in Infancy (hSMEI), an intractable childhood epilepsy accompanied by ataxia and other neurological deficits, is caused by heterozygous loss-of-function mutations in Nav1.1 channels. Because loss of Na channels would cause hypoexcitability, it is paradoxical that hSMEI is caused by haploinsufficiency of Nav1.1 channels. We have created a mouse model of SMEI by targeted deletion of Nav1.1 channels in mice, mSMEI. Like hSMEI, heterozygotes with mSMEI have intractable seizures after weaning, ataxia, and often premature death, which are strikingly dependent on genetic background as in humans. Our results reveal a potential basis for hyperexcitability--selective loss of Na currents in GABAergic inhibitory neurons compared to excitatory pyramidal neurons in the hippocampus. Moreover, a dramatic loss of Na current in GABAergic Purkinje neurons in the cerebellum may cause ataxia. Up-regulation of Nav1.3 channels is unable to compensate for loss of Nav1.1 channels. We will probe the cellular and molecular changes in function, localization, regulation, and cell biology of Na channels in mSMEI. We hypothesize that the epilepsy, ataxia, and other deficits in mSMEI result from loss of Na current in GABAergic inhibitory neurons, that genetic background effects on epileptogenesis in MSMEI are caused by differences in compensatory regulation of expression, localization, and function of other Na channels, and that novel combinations of anti-epileptic drugs that enhance GABAergic neurotransmission will be effective in treating mSMEI. Our experiments will be guided by four Specific Aims: (1) to determine whether loss of Na current in specific classes of GABAergic neurons is responsible for the pleiotropic effects of mSMEI; (2) to define the molecular basis for genetic background effects on severity of mSMEI due to expression, localization, and function of Na channels; (3) to describe and analyze the molecular and cellular changes in Na channels that lead from loss of excitability of GABAergic interneurons to hyperexcitability and epilepsy in mSMEI; and (4) to explore novel combination therapies for mSMEI that may be relevant for treatment of the human disease. Our results will provide crucial new information on the cell biology and regulation of Na channels in a relevant disease model, define the molecular and cellular mechanisms underlying mSMEI, and yield insights into novel pharmacotherapies that may be effective in this intractable childhood disease. PUBLIC HEALTH RELEVANCE: Severe Myoclonic Epilepsy in Infancy, an inherited seizure disorder cause by a gene defect in a sodium channel, is one of the most severe childhood epilepsy disorders. This project will use a mouse genetic model of this disease to determine how this gene defect causes epilepsy and to test new drug combinations for control of this intractable epilepsy syndrome.